The present disclosure relates to an electric power transmission apparatus which transmits electric power in a contactless manner, a contactless electric power transmission system, and a signal generation method.
In contactless electric power transmission (so-called wireless power feeding), it is necessary to output a high-power sine wave signal from an electric power transmission apparatus. An apparatus which generates such a sine wave signal is called a high-frequency power source. For example, it is easy to change the signal level of the sine wave signal which is output from the high-frequency power source in an audio power amplifier or the like which does not desire a high efficiency of power conversion from the high-frequency power source to the sine wave signal at a low frequency and low power. However, in the high-frequency power source used for the wireless power feeding, which desires a high efficiency of power conversion at a high frequency and high power, it is difficult to change the output electric power. Therefore, generally, an inverter is used which generates a square wave by performing a switching operation in which high efficiency can be acquired with a simple circuit configuration.
For example, a highly-efficient high-frequency power source which can be applied to a contactless electric power transmission system includes a class E amplifier (class E inverter), a half-bridge inverter, and a full-bridge inverter (so-called H bridge circuit).
The outline of an electric power transmission apparatus which uses a general high-frequency power source will be described with reference to FIG. 1. FIG. 1 is a circuit diagram schematically illustrating a general high-frequency power source which uses a half-bridge inverter.
The electric power transmission apparatus shown in FIG. 1 includes, as an example of the simplest configuration, for example, a high-frequency power source 101, and a resonance circuit 104 (series resonance circuit) which has an electric power transmission coil 102 and a resonance capacity 103. The high-frequency power source 101 uses a half-bridge inverter as a signal generator. Since the configuration of the half-bridge inverter is well known, the detailed circuit configuration thereof is omitted. The high-frequency power source 101 generates a square-wave signal as a high-frequency output signal, and supplies the square-wave signal to the resonance circuit 104.
However, every high-frequency power source is an inverter which generates a square wave using a switching method. The square wave which is the output signal includes a lot of harmonics, and relates to unnecessary radiation from the electric power transmission apparatus.
Japanese Unexamined Patent Application Publication No. 2011-120216 discloses an antenna driving apparatus that includes a trapezoidal wave signal generation circuit which generates a trapezoidal wave signal based on a square-wave signal at a predetermined frequency, and a trapezoidal wave signal amplifier circuit which amplifies the trapezoidal wave signal and supplies the amplified trapezoidal wave signal to an antenna negative load. In the antenna driving apparatus, the trapezoidal wave signal amplifier circuit supplies a normal phase output signal and a reverse phase output signal, the phases of which are reversed from each other, to both ends of the antenna negative load, and doubles a voltage which is applied to the negative load.